Adaptive filters operate on a signal supplied thereto in accordance with a prescribed criterion to generate a desired output signal. Typically, the filters generate an impulse response characteristic in accordance with an algorithm which includes updating of the impulse response characteristic in response to an error signal. In this way the filter characteristic is optimized to yield the desired result.
It has been found advantageous to normalize the update gain of the adaptive filter. The normalization tends to make the performance of the filter insensitive to variations in received signal power. In one prior arrangement an average of the squares of input signal sample magnitudes is used to normalize the gain, as described in an article by Mr. D. L. Duttweiler entitled "A Twelve-Channel Digital Echo Canceler", IEEE Transactions on Communications, Vol. COM-26, No. 5, May 1978, pp. 647-653. Another gain normalization arrangement employing a sum of the squares power estimate is disclosed in U.S. Pat. No. 3,922,505 issued Nov. 25, 1975. These prior arrangements attempt to protect against generating noisy, i.e., uncontrollably large, tap weights by controlling the update gain to be a "low" value for large incoming signal power estimate values.
Although these prior arrangements may perform satisfactorily in some applications, problems arise in others. For example, situations can arise where the open-loop gain of the circuit including the adaptive filter becomes greater than unity. This causes "singing", i.e., oscillating, to occur in the circuit which, in turn, causes large power estimate values. In such instances, the low gain value generated by the prior gain normalization arrangements would slow down the correction of the oscillating condition.
Another problem may arise in applications where two adaptive filters are employed as echo cancelers in a bidirectional voice frequency repeater when talking in both directions of transmission occurs simultaneously. This occurs, for example, for full duplex data set transmission. When talking occurs in both directions of transmission, the echo cancelers are typically inhibited from updating their impulse response characteristics and proper estimates of the echo signals are not obtained.